II. Physiology: Fetus
- Fetal gas exchange occurs at the placenta (not the lung)
- Ductus venosus
- Carries oxygenated blood from the placenta
- Delivers to the inferior vena cava via the umbilical vein
- Mixes with deoxygenated blood returning from the lower extremity
- Pulmonary arteries
- Returns blood from the descending aorta to the placenta to be re-oxygenated
- Ductus venosus
- Fetal right ventricular output
- Right to left shunt, bypassing the lungs (60% of flow)
- Increased pulmonary vascular resistance
- Ductus arteriosus
- Foramen ovale
- Pulmonary system (40% of flow)
- Alveoli are filled with amniotic fluid and result in minimal gas diffusion
- Right to left shunt, bypassing the lungs (60% of flow)
III. Physiology: Birth Transition from Fetal Circulation
- Alveolar fluid is replaced with air on initial ventilations
- Critical first step that leads to reduced pulmonary vascular resistance
- Results in an increase in PaO2 with an associated decreased in pulmonary vascular resistance
- Negative intrathoracic pressure increases
- Initiated with uterine contractions and travel through the birth canal
- First few breaths add to the negative intrathoracic pressure
- Increased pulmonary perfusion
- Results from less resistance to flow through pulmonary system than through fetal shunts
- Increased Systemic Vascular Resistance following delivery
- Flow reverses in the ductus arteriosus (now traveling aorta to pulmonary artery)
- Closure of shunts (last steps in transition)
- Ductus arteriosus closes within hours to days (unless Patent Ductus Arteriosus)
- Foramen ovale closes last
- Results from increased left atrial flow and pressure
-
Myocardium changes
- Right ventricle wall thins
- Left ventricle wall hypertrophies
IV. Resources
- Cove Point Foundation - Congenital Heart Disease
V. References
- Joseph and Webb (2015) Crit Dec Emerg Med 29(1): 10-8